Precipitation Hardenable Martensitic Stainless Steel

ABSTRACT

A precipitation hardenable stainless chromium nickel steel is disclosed having the following composition in weight %: 
     
       
         
               
               
               
             
                   
                   
               
                   
                 C 
                 max 0.07 
               
                   
                 Si 
                 max 1.5 
               
                   
                 Mn 
                 0.2-5   
               
                   
                 S 
                 0.01-0.4  
               
                   
                 Cr 
                 10-15 
               
                   
                 Ni 
                  7-14 
               
                   
                 Mo 
                 1-6 
               
                   
                 Cu 
                 1-3 
               
                   
                 Ti 
                 0.3-2.5 
               
                   
                 Al 
                 0.2-1.5 
               
                   
                 N 
                 max 0.1 
               
                   
                   
               
           
              
             
             
              
              
              
              
              
              
              
              
              
              
              
              
             
          
         
       
         
         
           
             balance Fe and normally occurring impurities.

The present disclosure is concerned with the precipitation hardenable stainless chromium nickel steels, more especially those that are hardenable with a fairly simple alloy treatment. More particularly, the concern is with the precipitation hardenable stainless chromium nickel steels which are hardened at a low temperature and also have good machinability when subjected to drilling, turning, milling and other cutting operations.

BACKGROUND AND PRIOR ART

Precipitation hardenable martensitic stainless steels are for example used in various high strength applications, such as springs, surgical needles, clips, fine tubes, parts for instruments and parts exposed to wear. In order to be a satisfactory material selection for such high strength applications the material needs to posses certain properties. For example, the steel should be able to be produced by an easy manufacturing process, including alloying, casting, hot working as well as cold working, rendering a high strength material that is easy to fabricate by mechanical cutting methods into finished parts starting from wire, sheet, strip, bar or tube material.

Also, despite the high final strength the material should preferably be ductile and thus allowing severe forming such as bending, coiling, pressing, twisting etc. in the as received condition. In addition to the mechanical properties above a very good corrosion resistance is often required, allowing the material to be used in different environments without having to consider additional corrosion protection such as painting or any other type of surface coating. Furthermore, the steel should be able to be hardened to a high final hardness by a simple, low temperature, alloy treatment that causes minimal shape disturbance.

Presently several alloys of different alloying concepts exits that covers many of the above mentioned requirements both by means of a trouble free production of parts as well as the properties required for the product itself during service. In several cases however, the properties required are difficult to combine in one single material. Carbon steels are, dependent on their composition, more or less formable and can with higher carbon contents be hardened to a high hardness. The low corrosion resistance makes however these steel impossible to use in environments that are even only slightly corrosive. Ferritic chromium steels may exhibit good corrosion resistance but cannot be hardened to a high strength. Martensitic chromium steels can be hardened but suffers from low ductility that limits their usage. Austenitic stainless steels are, dependent on their composition, either soft and ductile in the annealed condition or hard and less ductile in the cold deformed condition. In the harder conditions austenitic steels are also very difficult to machine. A further group is the precipitation hardenable stainless steels that can be formed in fairly soft condition and subsequently alloy treated to achieve a high hardness. Also this group of steels are more difficult to machine compared e.g. to the group of hardenable martensitic steels.

Consequently, it is need to provide the market with a material that satisfies the above criteria.

SUMMARY

A precipitation hardenable stainless chromium nickel steel is disclosed having the following composition in weight %:

C max 0.07 Si max 1.5 Mn 0.2-5   S 0.01-0.4  Cr 10-15 Ni  7-14 Mo 1-6 Cu 1-3 Ti 0.3-2.5 Al 0.2-1.5 N max 0.1

-   -   Balance Fe and normally occurring impurities.

The stainless steel according to the invention comprises titanium sulphides.

The stainless steel is hardened at a low temperature and has good machinability when subjected to drilling, turning, milling and other cutting operations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 SEM photograph of a reference alloy.

FIG. 2 a SEM photograph of FIG. 1 marked with test point Spectrum 1.

FIG. 2 b EDX result of Spectrum 1 of FIG. 2 a.

FIG. 3 a SEM photograph of FIG. 1 marked with test point Spectrum 2.

FIG. 3 b EDX result of Spectrum 2 of FIG. 3 a.

FIG. 4 a SEM photograph of FIG. 1 marked with test point Spectrum 3.

FIG. 4 b EDX result of Spectrum 3 of FIG. 4 a.

FIG. 5 SEM photograph of an alloy according to the invention.

FIG. 6 a SEM photograph of FIG. 5 marked with test point Spectrum 1.

FIG. 6 b EDX result of Spectrum 1 of FIG. 6 a.

FIG. 7 a SEM photograph of FIG. 5 marked with test point Spectrum 2.

FIG. 7 b EDX result of Spectrum 2 of FIG. 7 a.

FIG. 8 a SEM photograph of FIG. 5 marked with test point Spectrum 3.

FIG. 8 b EDX result of Spectrum 3 of FIG. 8 a.

FIG. 9 a SEM photograph of FIG. 5 marked with test point Spectrum 4.

FIG. 9 b EDX result of Spectrum 4 of FIG. 4 a.

DETAILED DESCRIPTION

In order to fully understand the influence of composition on the properties of the invented precipitation hardenable stainless steel it is necessary to discuss all elements individually. All element contents are in weight percent.

Carbon is a powerful element that affects the steel in many ways. A high carbon content will affect the deformation hardening in a way that the strength upon cold deformation will be high and thus reducing the ductility of the steel. A high carbon content is also disadvantageous from corrosion point of view as the risk of precipitation of chromium carbides increase with increasing carbon content. The carbon content should therefore be kept low, max 0.07%, preferably max 0.05% and more preferable max 0.025%.

Silicon is a ferrite-forming element and may also in higher contents reduce the hot working properties of the steel. The content should therefore be max 1.5% more preferably max 1.0%.

Manganese is an austenite-forming element that in a similar way as nickel makes the steel less prone to a martensitic transformation at cold deformation. The minimum content of manganese of the steel according to the invention is 0.2% by weight. As the steel has to have a significant content of martensite for the precipitation hardening the manganese content has to be max 5%, preferably max 3% and most preferably 2.5%. Manganese will together with sulfur form ductile non-metallic inclusions that for example are beneficial for the machining properties.

Sulfur is an element that will form sulfides in the steel. Sulfides are beneficial during machining as they will act as chip-breakers. The content of sulfur is therefore min 0.01% and more preferably min 0.015%, even more preferably 0.05% and most preferably min 0.1%. Sulfides may however act as weak areas in the steel from a corrosion resistance point of view. Further, high contents of sulfur may also be detrimental for the hot working properties. The content should therefore be max 0.4% and preferably max 0.3%. The composition of the alloy according to the invention is so selected that the alloy comprises titanium sulphides. The titanium sulphides are principally present in the form of TiS or Ti₂S.

Chromium is essential for the corrosion resistance and must in the steel according to the invention be added in a content of at least 10%, or more preferably at least 11.5%. Chromium is however also a strong ferrite former that in higher contents will suppress the martensite formation upon deformation. The content of chromium therefore has to be restricted to max 15%, preferably max 14%.

Nickel is added to the steel according to invention to balance the ferrite forming elements in order to obtain an austenitic structure upon annealing. Nickel is also an important element to moderate the hardening from cold deformation. Nickel will also contribute to the precipitation hardening together with elements such as titanium and aluminum. The minimum content of nickel is therefore 7% or more preferable at least 8%. A too high content of nickel will restrict the possibility to form martensite upon deformation. Nickel is also an expensive alloying element. The content of nickel is therefore maximized to 14 or preferably 13%.

Molybdenum is essential for the steel according to the invention, as it will contribute to the corrosion resistance of the steel. Molybdenum is also an active element during the precipitation hardening. The minimum content is therefore 1% or preferably, minimum 2% and most preferably minimum 3%. A too high content of molybdenum will however promote the formation of ferrite to a content that may result in problems during hot working. Further, a high content of molybdenum will also suppress the martensite formation during cold deformation. The content of molybdenum is therefore maximized to 6% and more preferable maximum 5%. Furthermore, it is expected that Mo could be partly or totally replaced by tungsten according to the common practice known to a person skilled in the art while still achieving the desired properties of the alloy.

Copper is an austenite former that together with nickel stabilizes the austenitic structure that is desired. Copper is also an element that increases the ductility in moderate contents. The minimum content is therefore 1% and more preferably at least 1.5%. On the other hand copper in high contents reduces the hot workability why the copper content is maximized to 3%, preferably maximum 2.5%.

Titanium is an essential alloying element in the invention due to at least two reasons. Firstly, titanium is used as a strong element for precipitation hardening and must therefore be present to be able to harden the steel for the final strength. Secondly, titanium will together with sulfur form titanium sulfides (TiS or possibly Ti₂S). In general, titanium is a stronger sulfide former than manganese. As TiS are electrochemically nobler that MnS it is possible to achieve improved machining properties without deterioration of the corrosion resistance that is the normal case for free machining steels that utilize MnS for the increased machinability. Therefore, the minimum content of titanium is 0.3% and more preferably 0.5%. Too high titanium contents will promote ferrite formation in the steel and also increase the brittleness. The maximum content of titanium should therefore be restricted to 2.5% preferably 2% and most preferably not more than 1.5%.

Aluminum is added to the steel in order to improve the hardening effect upon heat treatment. Aluminum is known to form intermetallic compounds together with nickel such as Ni3Al and NiAl. In order to achieve a good hardening response the minimum content should be 0.2% and most preferably min 0.3%. Aluminum is however a strong ferrite former why the maximum content should be 1.5% or more preferably max 1.0%.

Nitrogen is a powerful element as it will increase the strain hardening. However, it will also stabilize the austenite towards martensite transformation at cold forming. Nitrogen also has a high affinity to nitride formers such as titanium, aluminum and chromium. The nitrogen content should be restricted to maximum 0.1%, preferably 0.07% and most preferably max 0.05%.

According to one embodiment of the invention the alloy is substantially free of manganese sulfides.

The alloy of the invention has according to a preferred embodiment the following approximate composition in percent by weight:

C max 0.2 Si max 0.3 Mn 0.5 S 0.1 Cr 12 Ni 9 Mo 4 Cu 2 Ti 1 Al 0.4 N max 0.1

-   -   balance Fe and normally occurring impurities

According to another preferred embodiment of the invention, the alloy has the following approximate composition in percent by weight:

C max 0.2 Si max 0.3 Mn 2.5 S 0.1 Cr 12 Ni 9 Mo 4 Cu 2 Ti 1 Al 0.4 N max 0.1

-   -   balance Fe and normally occurring impurities

The present disclosure will now be described in more detail with the aid of some illustrative examples.

EXAMPLE 1

Five 270-kg melts, with chemical composition according to Table 1, were melted in a vacuum induction melting (VIM) furnace and cast into 9″, i.e. 229 mm, ingots. Alloy 830207 is included as a reference alloy and does not make a part of the present invention. The ingots were forged into 103×103 mm slabs. The slabs were then heated to 1150° C. and hot rolled to 5.5 mm wire rod. The wire rod was pickled and drawn to 2.1 mm in a multi block drawing machine without any intermediate annealing. The tensile strength in MPa as a function of the degree of area reduction at drawing of the hot rolled wire in diameter 5.5 mm to diameter 2.1 mm is found in Table 2. The tensile testing was made without any heat treatment and in accordance with SS-EN10002-1.

The compositions given in Table 1 of the alloy according to the invention can be classified as having a Mn content of approx. 0.5% or approx. 2.5% and a S content of approx. 0.015% or 0.1%.

TABLE 1 Chemical composition in weight percent Alloy C Si Mn S Cr Ni Mo Ti Cu Al N 830207 0.012 0.25 0.51 0.0026 12.07 9.13 4.06 0.97 2.05 0.41 0.009 830208 0.011 0.21 0.53 0.015 12.18 9.10 4.04 0.76 2.05 0.34 0.018 830209 0.011 0.21 0.44 0.092 12.10 9.01 4.02 0.80 2.02 0.31 0.006 830210 0.016 0.15 2.43 0.017 12.12 9.13 4.07 0.91 2.01 0.38 0.014 830211 0.014 0.20 2.55 0.093 11.97 9.04 4.04 0.90 2.00 0.40 0.009

TABLE 2 Tensile strength in MPa at various area reductions at cold drawing alloy Condition 830207 830208 830209 830210 830211 Wire rod 660 670 660 640 670 25.1% 850 850 870 905 900 42.5 1020 1025 1020 1110 1070 55.5 1170 1180 1190 1230 1190 65.5 1320 1310 1300 1320 1290 72.6 1430 1405 1380 1410 1420 77.8 1500 1475 1435 1470 1420 82.5 1585 1550 1510 1530 1500 85.6 1650 1610 1575 1590 1540

EXAMPLE 2

The drawn wires according to Table 2 were heat treated at 475° C. for 4 h and tensile tested in order to evaluate the increase in mechanical strength upon precipitation hardening (PH). The tensile strength after heat treatment resulting in the precipitation hardening can be seen in Table 3.

TABLE 3 Tensile strength in MPa after wire drawing at various reduction and subsequent precipitation hardening. alloy 830207- 830208- 830209- 830210- 830211- Condition PH PH PH PH PH Wire rod 664 675 658 670 669 25.1% 1030 1070 1172 955 932 42.5 1550 1575 1630 1220 1187 55.5 1903 1895 1900 1440 1396 65.5 2115 2055 2045 1635 1590 72.6 2265 2200 2130 1795 1759 77.8 2330 2275 2212 1955 1898 82.5 2450 2350 2270 2105 2034 85.6 2560 N/A N/A 2200 2154

EXAMPLE 3

The corrosion resistance, of the alloys according to Table 1, was tested in the precipitation hardened condition. The Critical Pitting Temperature (CPT) was measured in a 0.1% NaCl solution. The potential was kept on 300 mV Vs. Standard Calomel Electrode (SCE). After grinding to 600 grit finish 6 specimens per alloy in diameter 3.5 mm were tested to establish the critical pitting temperature of each material. Table 4 shows the CPT values per alloy.

TABLE 4 CPT values in 0.1% NaCl solution at 300 mV Vs SCE. 6 specimen tested alloy CPT, ° C., CPT, ° C. Alloy Average value of 6 specimens Standard deviation 830207 85 23 830208 76 29 830209 95 0 830210 95 0 830211 95 0

EXAMPLE 4

The machinability of the compositions according to Table 1 was also tested. Straightened bars in diameter 3.5 mm were manufactured by drawing wire from a diameter of 5.5 mm to 3.5 mm followed by a straightening operation. Drilling tests were performed in the as straightened, not heat treated, condition with mechanical properties according to Table 5. Hardness testing was made according to SS-EN ISO 6507.

TABLE 5 Dimension and mechanical properties of straightened bars used for machinability test. Hardness [HV5Kg] Diameter Half Yield strength Tensile strength Alloy [mm] Surfac Radius Centre [MPa] [MPa] 830207 3.504-3.504 371 367 367 1000-1000 1120-1130 830208 3.498-3.498 367 376 376 1010-1000 1150-1150 830209 3.495-3.498 367 376 376  990-1010 1180-1180 830210 3.510-3.510 353 362 367 980-960 1110-1110 830211 3.504-3.504 341 353 349 960-950 1090-1090

The drilling test was made with the drilling parameters shown in Table 6. The drills used were Ø2 mm uncoated cemented carbide drills, HAM 380, with a tip angle of 130°. Drilling depth was two times the bar diameter.

TABLE 6 Machining data at drilling Operation Cutting speed m/min Feeding mm/turn Drilling 5 0.05 Cutting off 10 (but max 4000 rpm) 0.005

The machinability was evaluated regarding chip formation, drill wear and drillability. The chip shape at drilling was judged by use of “Svenska Mekanforbundets Spauskala” (Karlebo Handbok, 15^(th) Edition, 2000, page 449-450) as a reference chart. The optimal chip formation for best productivity is No 5-7.

TABLE 7 Chip form at drilling. Alloy Chip form Rank 830207 The 2 first 7, remaining 2-3 3 830208 5-7 1 830209 5-7 1 830210 Single chips 6-7 remaining 1-3 3 830211 Some with irregular shape, generally 5-7 2

The wear was measured as the wear at clearance face on the cutting edge (flank wear), corner wear by built up edge and possible edge damages. The tests were in some cases performed for two samples of each alloy composition. The damages/wear was then graded after a scale with respect of the different types of wear/damages wherein as low of a grade as possible is desirable. A low value indicates a longer tool life compared to a high value. The results are disclosed in Table 8.

TABLE 8 Graded results of wear tests. Build up edge- Corner Grade Total rank Grade 1 = mild Flank wear, wear, 1 = mild 1 = best Alloy 3 = severe mm mm Chipping 3 = severe tool life 830207-1 Yes-3 0.1  N.a. Yes-flank 2 6 830207-2 Yes-3 — 0.08 Yes-corner 3 5 830208-1 Yes-2 — 0.09 No — 2 830208-2 No — 0.1  Yes-corner 3 4 830209-1 Yes-2 0.05 0.04 No — 3 830209-2 No 0.03 0.1  No — 2 830210-1 No 0.04 0.08 Yes-flank 3 5 830210-2 No 0.07 — No — 1 830211-1 Yes-3 0.05 — No — 1 830211-2 Yes-2 0.04 — No — 1

The ranking of the average of the tool wear of the two tests per alloy is seen in Table 9. A lower ranking indicates a longer tool life than a higher ranking.

TABLE 9 Tool wear at drilling, average of the two tests in Table 8. Alloy Ranking 830207 5 830208 3 830209 2 830210 3 830211 1

Large scale drilling tests were made on material from heat 830207 and 830209. The drillability was measured by means of the number of drilled parts until worn out drill. Also, the number of drilled holes per hour was measured. The results are shown in Table 10.

TABLE 10 Results of large scale drilling test Alloy Holes produced per drill Drilled holes per hour 830207 13700 1320 830209 23250 1680

The drilling tests show that all alloys according to the invention show an improved machinability compared to the reference alloy 830207 in form of chip formation that is important for minimizing the risk of chips tangling during drilling. All alloys according to the invention also show less tool wear compared to the reference alloy 830207, which means that more parts can be produced before the drill must be replaced.

EXAMPLE 5

The reference alloy 830207 and the 830211 alloy according to the invention were analyzed by means of Scanning Electron Microscope (SEM) using Back Scattered Electrons (BSE). The surfaces of the materials were in un-etched condition. A photograph of the reference alloy taken in the SEM is shown in FIG. 1. Three different test points; Spectrum 1 illustrated in FIG. 2 a, Spectrum 2 illustrated in FIG. 3 a and Spectrum 3 illustrated in FIG. 4 a; were investigated by Energy Dispersive X-ray analysis (EDX). The results are shown in FIGS. 2 b, 3 b, and 4 b, respectively. As can be clearly seen from the results, there are no titanium sulfides. This is considered to be a result of the low sulphur content of the alloy.

A photograph of the 830211 alloy of the invention taken in the SEM is shown in FIG. 5. The difference can be clearly seen when comparing to FIG. 1. Four different test points; Spectrum 1, Spectrum 2, Spectrum 3 and Spectrum 4; are given in the FIGS. 6 a, 7 a, 8 a, and 9 a. The composition of these test points were analyzed with EDX and the results are given in FIGS. 6 b, 7 b, 8 b and 9 b, respectively.

Surprisingly, no manganese sulphides were found in the alloy 830211 even though the M content was quite high (approximately 2.5%).

It is clear form the Examples that a new alloy has been developed which has substantially improved machining properties without a reduced corrosion resistance in comparison with the previously known alloy. It is likely that increasing the sulfur content further would increase the machinability further without reducing the corrosion resistance as long as the content of manganese sulfides are not substantially increased. 

1: Martensitic stainless steel alloy characterized in having the following composition in weight-%: C [[max 0.07]] max 0.07 Si [[max 1.5]] max 1.5 Mn [[0.2-5]] 0.2-5 S [[0.01-0.4]] 0.01-0.4 Cr 10-15 Ni 7-14 Mo 1-6 Cu 1-3 Ti [[0.3-2.5]] 0.3-2.5 Al [[0.2-1.5]] 0.2-1.5 N [[max 0.1]] max 0.1

balance Fe and normally occurring impurities wherein the alloy comprises titanium sulphides. 2: Martensitic stainless steel alloy according to claim 2 wherein the alloy is substantially free from manganese sulfides. 3: Martensitic stainless steel alloy according to claim 1 wherein the content of S is 0.015-0.3%. 4: Martensitic stainless steel alloy according to claim 1 wherein the content of Ti is at least 0.5%. 5: Martensitic stainless steel alloy according to claim 4 wherein the content of Ti is max 2%. 6: Martensitic stainless steel alloy according to claim 1 wherein the content of Mo is 2-5%. 7: Martensitic stainless steel alloy according to claim 1 wherein the content of Cr is 11.5-13% and the content of Ni is 8-13%. 8: Martensitic stainless steel alloy according to claim 1 wherein the alloy is precipitation hardened. 9: Martensitic stainless steel alloy according to claim 1 wherein the alloy is produced by conventional metallurgy techniques. 10: Martensitic stainless steel alloy according to claim 1 wherein the alloy has an approximate composition of C [[max 0.2]] max 0.2 Si [[max 0.3]] max 0.3 Mn [[0.5]] 0.5 S [[0.1]] 0.1 Cr 12  Ni 9 Mo 4 Cu 2 Ti 1 Al [[0.4]] 0.4 N [[max 0.1]] max 0.1

balance Fe and normally occurring impurities. 11: Martensitic stainless steel alloy according to claim 1 wherein the alloy has an approximate composition of C [[max 0.2]] max 0.2 Si [[0.2]] 0.2 Mn [[2.5]] 2.5 S [[0.1]] 0.1 Cr 12  Ni 9 Mo 4 Cu 2 Ti 1 Al [[0.4]] 0.4 N [[max 0.1]] max 0.1

balance Fe and normally occurring impurities. 12: Martensitic stainless steel alloy according to claim 10 wherein the alloy is substantially free from manganese sulfides. 13: Martensitic stainless steel alloy according to claim 11 wherein the alloy is substantially free from manganese sulfides. 14: Martensitic stainless steel alloy according to claim 2 wherein the content of S is 0.015-0.3%. 15: Martensitic stainless steel alloy according to claim 2 wherein the content of Ti is at least 0.5%. 16: Martensitic stainless steel alloy according to claim 15 wherein the content of Ti is max 2%. 17: Martensitic stainless steel alloy according to claim 3 wherein the content of Ti is at least 0.5%. 18: Martensitic stainless steel alloy according to claim 17 wherein the content of Ti is max 2%. 